Hypoxemia

Background

Defined as PaO2 <60 mm Hg
- Amount of oxygen available is a function of the arterial oxygen content and blood flow
Occurs in states of low CO, low hemoglobin, or low SaO2 (arterial oxygen saturation)
Relative Hypoxemia
- PaO2 is < expected for given level of inhaled O2 (e.g. PaO2 of 100 on FiO2 100%)
- Can calculate amount of relative hypoxemia by A-a gradient
  - P(A-a)O2 = 145 – PaCO2 – PaO2 (normal is <10 in young, healthy patients)

Hypoventilation
- Always associated with increased PaCO2
- Normal A-a gradient
Right-to-left shunt
- Occurs when blood enters the systemic arteries with out traversing ventilated lung
  - Occurs with pulmonary consolidation, atelectasis
- Hallmark is failure of arterial O2 levels to increase with supplemental O2 (increased A-a)
V/Q mismatch
- PE, pneumonia, asthma, COPD
- Improves with supplemental O2
- A-a gradient increased
Diffusion impairment
- Improves with supplemental O2
- A-a gradient increased
Low inspired O2
- Improves with supplemental O2
- A-a gradient normal